Pigs, Curlews and Trains: Geophysical Survey

New surveys south of the A303 extend the very effective existing coverage to the north.

Geophysical survey has proved highly effective throughout the Stonehenge World Heritage Site, but the physical separation of the northern and southern parts of the site is to some extent reflected in the current distribution of survey coverage. The A303 marks a transition from the accessible and predominantly pastoral land surrounding Stonehenge itself to the more varied regime in the south, with its arable land, pig farming, and RSPB stone curlew reserve. Physical access to the southern World Heritage Site for geophysical survey can be difficult, given the need to work around agricultural constraints and the breeding season of the ground-nesting birds.

Despite these restrictions the southern World Heritage Site is a rich archaeological landscape, in which the many fascinating known monuments pose a series of questions about how sites interrelate and the completeness of our knowledge of the landscape as a whole.

While some specific monuments and areas have been targeted in the past there is clearly much to be obtained by ensuring there is detailed geophysical coverage of the entire World Heritage Site.

For the current project the authors embarked on a four-week programme of fieldwork, timed to make best use of limited access to part of the pig farm and the bird reserve over the winter. A combination of vehicle-towed geophysical instruments was deployed, resulting in coverage of 137 hectares of land with a high-sensitivity caesium magnetometer array and 48 hectare with high density, multi-channel ground penetrating radar (GPR). Measurements were taken at very small intervals – just a few centimetres apart in the case of the GPR – to ensure high resolution datasets, and co-mounted Global Navigation Satellite System receivers were used to ensure each reading was accurately positioned.

All of this data needed to be rapidly processed, interpreted and reported so that targeted excavation of any significant anomalies could follow immediately.

Invisible sites

Many such anomalies relate to known monuments or cropmarks identified from aerial photographs. Geophysical survey is able to confirm the location and survival of these archaeological remains, which may not be regularly visible from the air owing to crop rotation and changing climatic conditions, and provide additional information by seeing beneath the soil to detect details not visible on the surface. For example, aerial photography had suggested the possible existence of a series of closely-spaced or interlocking sub-circular ring-ditches within a group of three scheduled bowl barrows on the western side of the WHS.

A brief window of access to one of the fields used by the pig farm in this area allowed a magnetic survey to take place. Although such subtle archaeology often struggles to survive, the geophysics established the number of ring-ditches present, enhanced our knowledge of their form and revealed a more strongly magnetised linear ditch section nearby. The ditch crossed a modern field boundary into an adjacent field where it appeared to terminate without apparent explanation, making it an intriguing target for subsequent excavation.

The more recent history of the WHS is reflected in the path of the military light railway seen in the figure above, running parallel with the A360 and passing through the circular ditch of a possible henge monument first identified in a geophysical survey of the 1990s. Here, and despite the interference from the railway, the 2015 survey found good evidence for an entrance gap to the north-east, but no indication of internal activity. However, a group of pits was identified immediately to the north, and considered to possibly be associated with the ring-ditch.

The survey also covered the site of Coneybury Henge to the east of the WHS. Here a survey had been conducted in 1980 prior to the partial excavation of the monument, and it was possible to make a useful comparison with the results of a new survey using modern techniques.

The 1980 survey used fluxgate gradiometer technology; the 2015 one high-sensitivity caesium coverage. The figure below shows that the new data replicates the previously recorded curvilinear anomalies over the henge ditch, with an equal magnitude of response and a similar entrance gap to the north-east. A less pronounced break in the ditch to the south, absent from the original survey data, marks the location of an excavation trench. The presence of intense internal magnetic anomalies, only mapped by the current survey, also seems most likely to have a more recent origin, possibly the excavation itself.

The eastern side of the WHS also produced a wealth of linear anomalies in both the magnetic and GPR datasets. These appear to indicate a network of ditches not fully represented in aerial photography. One of these, shown in the figure below, featured a bifurcation that was especially interesting given its location on high ground overlooking Stonehenge about 1 kilometre away.

Subsequent excavation of this ditch proved particularly fruitful, revealing two Middle Bronze Age burials and suggesting that these linear anomalies, some represented by quite subtle responses in the GPR data, define a network of boundaries demarcating the prehistoric landscape.

New detail on the North Kite

The survey also investigated the barrows and other large earthwork monuments found across Normanton Down, taking advantage of the opportunity for out-of-season access to the RSPB nature reserve. Although access was partially limited by such features as the remains of outdoor pig-rearing units, some new insights were revealed into the so-called North Kite, an area of well-preserved Bronze Age linear earthworks.

The figure below shows a broad linear magnetic anomaly following the known course of the North Kite ditch, together with a more subtle response running parallel to it, possibly representing an outer palisade. The northern segment of the North Kite ditch appears to be incorporated into an approximately east-west ditch anomaly which has been traced by aerial photography across the wider landscape and is thought to form a boundary around the Normanton Down barrow group. The GPR data replicates the magnetic responses to both the North Kite ditch and the palisade, and, in places where the magnetic data has been obscured by ferrous detritus, provides some improved definition of the linear earthworks. Greater complexity is also revealed within the northern arc of the Kite, where a series of evenly spaced buried stones or pits have been detected which cannot be entirely explained by the more recent activity shown on aerial photographs.

Between the known monuments the survey has revealed a plethora of small, discrete anomalies related to pits and tree throws. While these may not all be of archaeological significance, recent excavations within the WHS have demonstrated that such features can preserve important information about past activity in the Stonehenge landscape. Detailed analysis of the combined geophysical data may make it possible to further enhance interpretation of these results. The GPR data has also revealed new insights into the underlying geomorphology, particularly weathering bands within the chalk and the accumulation of sediments within dry valleys.

Landscape-scale geophysical survey is increasingly recognised as an important research and management tool within the World Heritage Site. For example, as shown below, it can determine the scale of animal burrows within barrows. Whether it is used to discover individual monuments, or to determine relationships between sites across the landscape, the new survey data serves as a useful contribution to a long hoped-for aim: complete coverage of the WHS using remote sensing survey techniques.

The authors

Paul Linford

Paul Linford MSc has worked as an archaeological scientist for English Heritage and Historic England since the mid-1980s and is head of the Geophysics Team. He has particular interests in archaeomagnetic dating and in developing the team’s caesium magnetometer array. Paul is also Treasurer of the International Society for Archaeological Prospection and a member of the Geological Society’s Near-Surface Geophysics Group committee.

Andy Payne

Andy Payne has specialised in the practice of archaeological geophysics since the early 1990s, working widely across England and occasionally in France, Spain and the Channel Islands. He has contributed to numerous reports and publications including, in 1995, the first published detailed geophysical investigations of Stonehenge and its immediate landscape setting (Payne 1995). His archaeological career has also included working on excavations in Orkney and the site of the Roman amphitheatre in London.

Neil Linford

Neil Linford PhD has been an archaeological geophysicist for over 25 years, and has experience across a wide range of applied techniques. Whilst his PhD focused on the magnetic properties of archaeological sediments, he also has expertise in all aspects of the use of GPR. He is an editor of the journal Archaeological Prospection and chair of the NERC Geophysical Equipment and Geodesy Facility steering committee.